Field of the Invention
The present invention concerns techniques for time-dependent intensity correction of diffusion-weighted magnetic resonance MR images that are acquired with a sequence of different diffusion gradient fields.
Description of the Prior Art
In routine clinical settings, diffusion-weighted magnetic resonance (MR) images can provide important diagnostic information, for example in stroke and tumor diagnostics. In diffusion-weighted MR imaging, diffusion gradient fields are switched (activated) in defined directions, and the resulting diffusion of water molecules along the applied diffusion gradient fields typically weakens the measured MR signal. In areas with lesser (greater) diffusion, a smaller (larger) signal attenuation thus typically takes place, such that these areas can have an increased (reduced) signal value in an imaging MR measurement.
The strength of the diffusion weighting can be correlated with the strength of the applied diffusion gradient fields; stronger (weaker) diffusion gradient fields can typically produce a stronger (weaker) diffusion weighting of the MR images. For example, the strength of the diffusion can be derived from the diffusion weighting so a quantitative conclusion can be made. In principle, it can be desirable to achieve a particularly strong or, large diffusion weighting, since the signal-to-noise ratio (SNR) in the diffusion-weighted MR images is then particularly high. For example, a physical “diffusion” measurement variable acquired with a poor SNR can result in a lower diagnostic certainty. Clinical applications can be implemented particularly precisely or so as to be less error-prone with a high SNR.
Moreover, techniques are known that—in addition to the spatially resolved information about the strength of the diffusion—provide information about the orientation or direction of the diffusion and/or the strength of the diffusion, resolved for different spatial directions. This is in contrast to techniques that have no direction dependency of the diffusion and, for example, provide the diffusion proportional to a value averaged over the different spatial directions. For example, this can take place by means of known Diffusion Tensor Imaging (DTI) or “High Angular Resolution Diffusion Imaging” (HARDI) techniques. See, for instance, “High Angular Resolution Diffusion Imaging Reveals Intravoxel White Matter Fiber Heterogeneity” by D. S. Tuch et al. in Mag. Reson. Med. 48 (2002) 577-582. While such techniques take into account a variety of different models for angle-resolved determination of the diffusion, a common factor is that a number of differently oriented diffusion gradient fields are used to acquire the MR images. In addition to different orientations of the diffusion gradient fields, different strengths of the diffusion gradient fields can also be taken into account. Typically known measurement sequences include, for example, 30-100 diffusion gradient fields with different orientation and/or strength; the implementation of the entire diffusion imaging can then require a duration of multiple minutes to multiple hours, typically between 5 and 20 min.
Therefore, such an MR imaging can result in a significant heating of gradient coils that are used to generate the diffusion gradient fields and the spatially coding gradient fields. This is because the diffusion gradient fields for diffusion imaging can have relatively high amplitudes, long time durations and frequent repetition. In typical MR systems, depending on the structure, the heat generated by such gradients can be transferred to the radio-frequency (RF) coils—for instance an RF body coil—that are used to radiate RF pulses to deflect the magnetization out of the steady state and/or to detect an MR signal. The heating of the RF coils can result in a change of the electrical properties of the RF coil, for example due to a direct temperature dependency of the electrical components and/or due to a geometric deformation of the RF coil due to the heating (for example thermal expansion). Therefore, a time dependency of the electrical properties of the RF coil during an MR measurement can be present due to the time-dependent heating of the components of the MR system. In particular, a time-dependent amplitude of the RF pulses can result due to the heating of the RF coils, which in turn can lead to a variation of the readout angle or flip angle of the excited magnetization. This can lead to a time-dependent intensity fluctuation in the diffusion-weighted MR images.
In modern MR systems with a large tunnel opening, a particularly close spatial proximity between the gradient system and the RF coils can exist. Therefore, such MR systems can be particularly sensitive to the above-described problem of heating during diffusion imaging. Intensity fluctuations in the range of a few percent to a few tens of a percent are known to occur.
Such time-dependent intensity fluctuations can also be designated as drift and are generally not desirable because, for example, the defined diffusion can thereby be adulterated. Such intensity fluctuations in the diffusion-weighted MR images can also be designated as signal errors, for example. An ability to assess the MR images for downstream applications can be limited due to the signal error.
In order to reduce these intensity fluctuations, solutions are known (for example) that monitor the amplitude of the RF pulses in a control/regulation loop. However, such a solution typically requires additional components, such that a corresponding implementation can be error-prone and expensive.